1. Field of the Invention
The present invention relates to a solid electrolytic capacitor employing a porous sintered body made of a valve-action metal, and to a method of making such a solid electrolytic capacitor.
2. Description of the Related Art
Electrolytic capacitors employing a porous body of a metal having a valve action (hereinafter simply ‘valve metal’) have been widely used in the past for purposes such as eliminating noise generated by a CPU or other device and stabilizing the power supply of electronic appliances.
An example of one method of making a conventional electrolytic capacitor of this type is provided in FIGS. 12 and 13 (cf. JP-A 10-106898). As shown in FIG. 12, the apparatus used in this method comprises a mold B1 containing a fixed block 101, a moveable block 102, and a pressure block 103. In this method, the space formed between fixed block 101 and moveable block 102 is filled with valve metal powder 104. Next, as shown in FIG. 13, a porous body 106 is formed by applying downward pressure to the powder 104 by lowering pressure block 103, into which a wire 105 has been set so as to protrude. This porous body is then fired and a porous sintered body obtained, from which the solid electrolytic capacitor is made.
In the method described above, the compression of the powder 104 takes place along the axis of the wire 105 during the pressurization process. As a result, there is a risk that the powder 104 will lack sufficient adhesiveness relative to the wire 105, and that the wire 105 and the porous body 106 will not bond satisfactorily.
Another method of making a solid electrolytic capacitor is shown in FIG. 14 (cf. JP-A 10-106897). The apparatus employed in this method comprises a mold B2 containing a fixed block 201, a moveable block 202, and a pair of pressure blocks 203. In this method, the space formed between fixed block 201 and the two pressure blocks 203 is filled with valve metal powder. This space is then covered with moveable block 202, into which a wire 205 has been set so as to protrude. After this, pressure is applied on the powder in a horizontal direction by means of the two pressure blocks 203, and a porous body is formed. With this method, the powder is compressed in a direction intersecting with the axis of wire 205. As a result, the powder is able to adhere satisfactorily to the wire 205.
Recent years have seen the development of CPUs with higher clock rates, and faster and digitized electronic appliances. In line with this, various new demands are being made for capacitors. For example, there is a need for improved noise elimination characteristics in broad frequency bands. There is also a growing demand for a highly responsive large-capacitance power supply supporting high frequencies. Making capacitors with larger capacitance, lower resistance, and lower impedance would be an effective way of responding to these demands. One desirable way of achieving this in the case of solid electrolytic capacitors comprising a porous sintered body would be to produce porous bodies that are larger, of a greater density, and also flatter than those in use heretofore.
The conventional method illustrated in FIG. 14, however, is unsuitable for making large, high-density, flattened porous bodies, for the reasons given below.
In order to make a larger, denser porous body by this method, it is necessary to increase the pressure applied on the valve metal powder, and also to increase the distance moved by the pressure blocks 203 as they compress the powder. For this purpose, a large-output drive source must be provided for each of the pressure blocks 203 (two drive sources in total). These two drive sources are installed opposite one another with the mold B2 between them while being spaced from one another in a horizontal direction. With this setup in place, however, the whole apparatus takes up too much space. Also, the need for two separate large-output drive sources is hardly desirable from the point of view of keeping costs low.